TkGetVRootGeometry returns geometry information about the virtual root window associated with tkwin.
The ``associated'' virtual root is the one in which tkwin's nearest top-level ancestor (or tkwin itself if it is a top-level window) has been reparented by the window manager.
This window is identified by a __SWM_ROOT or __WM_ROOT property placed on the top-level window by the window manager.
If tkwin is not associated with a virtual root (e.g.
because the window manager doesn't use virtual roots) then *xPtr and *yPtr will be set to 0 and *widthPtr and *heightPtr will be set to the dimensions of the screen containing tkwin.